Multi-threshold sensing of cardiac electrical signals in an extracardiovascular implantable cardioverter defibrillator

ABSTRACT

An implantable medical device system capable of sensing cardiac electrical signals includes a sensing circuit, a therapy delivery circuit and a control circuit. The sensing circuit is configured to receive a cardiac electrical signal and sense a cardiac event in response to the signal crossing a cardiac event sensing threshold. The therapy delivery circuit is configured to deliver an electrical stimulation therapy to a patient&#39;s heart via the electrodes coupled to the implantable medical device. The control circuit is configured to control the sensing circuit to set a starting value of the cardiac event sensing threshold and hold the starting value constant for a sense delay interval. The control circuit is further configured to detect an arrhythmia based on cardiac events sensed by the sensing circuit and control the therapy delivery circuit to deliver the electrical stimulation therapy in response to detecting the arrhythmia.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/054,089 (U.S. Publication No. 2018/0339164), now U.S. Pat. No.10,493,291, filed Aug. 3, 2018, which is a Division of U.S. patentapplication Ser. No. 15/142,171, now U.S. Pat. No. 10,252,071, filedApr. 29, 2016, the entire content of both of which is incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The disclosure relates generally to an extra-cardiovascular implantablecardioverter defibrillator (ICD) system, device and method for sensingcardiac electrical signals using extra-cardiovascular electrodes.

BACKGROUND

Medical devices, such as cardiac pacemakers and ICDs, providetherapeutic electrical stimulation to a heart of a patient viaelectrodes carried by one or more medical electrical leads and/orelectrodes on a housing of the medical device. The electricalstimulation may include signals such as pacing pulses or cardioversionor defibrillation shocks. In some cases, a medical device may sensecardiac electrical signals attendant to the intrinsic or pacing-evokeddepolarizations of the heart and control delivery of stimulation signalsto the heart based on sensed cardiac electrical signals. Upon detectionof an abnormal rhythm, such as bradycardia, tachycardia or fibrillation,an appropriate electrical stimulation signal or signals may be deliveredto restore or maintain a more normal rhythm of the heart. For example,an ICD may deliver pacing pulses to the heart of the patient upondetecting bradycardia or tachycardia or deliver cardioversion ordefibrillation shocks to the heart upon detecting tachycardia orfibrillation. The ICD may sense the cardiac electrical signals in aheart chamber and deliver electrical stimulation therapies to the heartchamber using electrodes carried by transvenous medical electricalleads. Cardiac signals sensed within the heart generally have a highsignal strength and quality for reliably sensing cardiac electricalevents, such as R-waves. In other examples, a non-transvenous lead maybe coupled to the ICD, in which case cardiac signal sensing presents newchallenges in accurately sensing cardiac electrical events.

SUMMARY

In general, the disclosure is directed to techniques for controlling acardiac event sensing threshold by a medical device. The cardiac eventsensing threshold may be an R-wave sensing threshold controlled by themedical device to avoid oversensing of T-waves and P-waves whilemaintaining high sensitivity for detecting ventricular tachyarrhythmia.In some examples, the medical device is an ICD coupled to anextra-cardiovascular lead carrying at least one sensing electrode.

In one example, the disclosure provides a system comprising animplantable medical device for sensing cardiac electrical events. Thesystem includes a sensing circuit, a therapy delivery circuit and acontrol circuit. The sensing circuit is configured to receive a cardiacelectrical signal from electrodes coupled to the implantable medicaldevice and sense a cardiac event attendant to a myocardialdepolarization in response to the cardiac electrical signal crossing acardiac event sensing threshold. The therapy delivery circuit isconfigured to deliver an electrical stimulation therapy to a patient'sheart via the electrodes coupled to the implantable medical device. Thecontrol circuit is configured to determine a gain based on a minimumthreshold value of the cardiac event sensing threshold, determine amaximum limit of a starting threshold value based on the gain and theminimum threshold value, and control the sensing circuit to set astarting value of the cardiac event sensing threshold to be equal to orless than the maximum limit. The control circuit is further configuredto detect an arrhythmia based on cardiac events sensed by the sensingcircuit and control the therapy delivery circuit to deliver theelectrical stimulation therapy in response to detecting the arrhythmia.

In another example, the disclosure provides a method performed by asystem including an implantable medical device for sensing cardiacelectrical signals. The method includes receiving a cardiac electricalsignal by a sensing circuit of the implantable medical device viaelectrodes coupled to the implantable medical device; setting a cardiacevent sensing threshold to a starting threshold value; sensing a cardiacevent attendant to a myocardial depolarization in response to thecardiac electrical signal crossing the cardiac event sensing threshold;detecting an arrhythmia based on a plurality of cardiac events sensed bythe sensing circuit; and controlling a therapy delivery circuit todeliver an electrical stimulation therapy in response to detecting thearrhythmia. Setting the starting threshold value includes determining bya control circuit of the implantable medical device a gain based on aminimum threshold value of the cardiac event sensing threshold,determining a maximum limit of the starting threshold value based on thegain and the minimum threshold value, and setting the starting value ofthe cardiac event sensing threshold to be equal to or less than themaximum limit.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a processor of an implantable medical device, cause thesystem to set a cardiac event sensing threshold to a starting thresholdvalue; sense a cardiac event attendant to a myocardial depolarization bya sensing circuit of the implantable medical device in response to acardiac electrical signal crossing the cardiac event sensing threshold;detect an arrhythmia based on a plurality of cardiac events sensed bythe sensing circuit; and control a therapy delivery circuit of theimplantable medical device to deliver the electrical stimulation therapyin response to detecting the arrhythmia. Setting the cardiac eventsensing threshold to the starting threshold value comprises determininga gain based on a minimum threshold value of the cardiac event sensingthreshold, determining a maximum limit of the starting threshold valuebased on the gain and the minimum threshold value; and setting thestarting value of the cardiac event sensing threshold to be equal to orless than the maximum limit.

In yet another example the disclosure provides a system comprising animplantable medical device for sensing cardiac electrical eventsincluding a sensing circuit, a therapy delivery circuit and a controlcircuit. The sensing circuit is configured to receive a cardiacelectrical signal from electrodes coupled to the implantable medicaldevice and sense a cardiac event attendant to a myocardialdepolarization in response to the cardiac electrical signal crossing acardiac event sensing threshold. The therapy delivery circuit configuredto deliver an electrical stimulation therapy to a patient's heart viathe electrodes coupled to the implantable medical device. The controlcircuit is configured to control the sensing circuit to: set the cardiacevent sensing threshold to a starting value; hold the cardiac eventsensing threshold constant at the starting value for a sense delayinterval; adjust the cardiac event sensing threshold to a secondthreshold value upon expiration of the sense delay interval; and holdthe cardiac event sensing threshold constant at the second thresholdvalue for a drop time interval. The control circuit is furtherconfigured to detect an arrhythmia based on a plurality of cardiacevents sensed by the sensing circuit; and control the therapy deliverycircuit to deliver the electrical stimulation therapy in response todetecting the arrhythmia.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a conceptual diagram of a distal portion of anextra-cardiovascular lead having an electrode configuration according toanother example.

FIG. 4 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 5 is a timing diagram showing a filtered and rectified cardiacelectrical signal and R-wave sensed event signals produced by a sensingcircuit of the ICD of FIG. 4.

FIG. 6 is a diagram of a filtered and rectified cardiac electricalsignal and a maximum sensing threshold limit according to one example.

FIG. 7 is a diagram of the cardiac electrical signal shown in FIG. 6 andmaximum sensing threshold limits determined based on a variable,sensitivity-dependent gain.

FIG. 8A is a diagram of a multi-level R-wave sensing threshold accordingto another example.

FIG. 8B is a diagram of a non-monotonic, multi-level R-wave sensingthreshold according to another example.

FIG. 9 is a flow chart of a method performed by the ICD of FIGS. 1A-2Cfor controlling the R-wave sensing threshold according to one example.

FIG. 10 is a flow chart of a method for selecting a sensitivity settingin the ICD system of FIGS. 1A-2C according to one example.

FIG. 11 is a flow chart of a method for selecting R-wave sensingthreshold control parameters according to one example.

FIG. 12 is a plot of an example of a ventricular tachycardia/ventricularfibrillation (VT/VF) detection sensitivity curve.

FIG. 13 is a plot of a VT/VF detection sensitivity curve as a functionof the programmed sensitivity setting.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for sensing cardiacelectrical signals using implanted, extra-cardiovascular electrodes. Asused herein, the term “extra-cardiovascular” refers to a positionoutside the blood vessels, heart, and pericardium surrounding the heartof a patient. Implantable electrodes carried by extra-cardiovascularleads may be positioned extra-thoracically (outside the ribcage andsternum) or intra-thoracically (beneath the ribcage or sternum) butgenerally not in intimate contact with myocardial tissue. The techniquesdisclosed herein provide a method for reliably sensing R-waves,attendant to ventricular depolarization, using extra-cardiovascularelectrodes by applying multiple sensing thresholds to avoid oversensingof T-waves attendant to ventricular repolarization and P-waves attendantto atrial depolarization.

The techniques are described in conjunction with an implantable medicallead carrying extra-cardiovascular electrodes, but aspects disclosedherein may be utilized in conjunction with other cardiac electricalsensing lead and electrode systems. For example, the techniques forcontrolling an R-wave sensing threshold as described in conjunction withthe accompanying drawings may be implemented in any implantable orexternal medical device enabled for sensing cardiac electrical signals,including implantable pacemakers, ICDs or cardiac monitors coupled totransvenous or epicardial leads carrying sensing electrodes; leadlesspacemakers, ICDs or cardiac monitors having housing-based sensingelectrodes; and external pacemakers, defibrillators, or cardiac monitorscoupled to external, surface or skin electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as a housing electrode (sometimes referred to as a canelectrode). In examples described herein, housing 15 may be used as anactive can electrode for use in delivering cardioversion/defibrillation(CV/DF) shocks or other high voltage pulses delivered using a highvoltage therapy circuit. In other examples, housing 15 may be availablefor use in delivering unipolar, low voltage cardiac pacing pulses inconjunction with lead-based cathode electrodes. In other instances, thehousing 15 of ICD 14 may include a plurality of electrodes on an outerportion of the housing. The outer portion(s) of the housing 15functioning as an electrode(s) may be coated with a material, such astitanium nitride.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,sensors, electrical cardiac signal sensing circuitry, therapy deliverycircuitry, power sources and other components for sensing cardiacelectrical signals, detecting a heart rhythm, and controlling anddelivering electrical stimulation pulses to treat an abnormal heartrhythm.

Lead 16 includes an elongated lead body 18 having a proximal end 27 thatincludes a lead connector (not shown) configured to be connected to ICDconnector assembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead 16 includes defibrillation electrodes 24 and 26 andpace/sense electrodes 28, 30 and 31. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently. In some instances, defibrillation electrodes 24and 26 are coupled to electrically isolated conductors, and ICD 14 mayinclude switching mechanisms to allow electrodes 24 and 26 to beutilized as a single defibrillation electrode (e.g., activatedconcurrently to form a common cathode or anode) or as separatedefibrillation electrodes, (e.g., activated individually, one as acathode and one as an anode or activated one at a time, one as an anodeor cathode and the other remaining inactive with housing 15 as an activeelectrode).

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to low voltage pacing and sensing electrodes 28, 30 and31. However, electrodes 24 and 26 and housing 15 may also be utilized toprovide pacing functionality, sensing functionality or both pacing andsensing functionality in addition to or instead of high voltagestimulation therapy. In this sense, the use of the term “defibrillationelectrode” herein should not be considered as limiting the electrodes 24and 26 for use in only high voltage cardioversion/defibrillation shocktherapy applications. Electrodes 24 and 26 may be used in a pacingelectrode vector for delivering extra-cardiovascular pacing pulses suchas anti-tachycardia pacing (ATP) pulses or bradycardia pacing pulsesand/or in a sensing vector used to sense cardiac electrical signals anddetect ventricular tachycardia (VT) and ventricular fibrillation (VF).

Electrodes 28, 30 and 31 are relatively smaller surface area electrodesfor delivering low voltage pacing pulses and for sensing cardiacelectrical signals. Electrodes 28, 30 and 31 are referred to aspace/sense electrodes because they are generally configured for use inlow voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals.In some instances, electrodes 28, 30 and 31 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. A third pace/senseelectrode 31 may be located distal to defibrillation electrode 26.Electrodes 28 and 30 are illustrated as ring electrodes, and electrode31 is illustrated as a hemispherical tip electrode in the example ofFIGS. 1A and 1B. However, electrodes 28, 30 and 31 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, hemispherical electrodes, directional electrodes,segmented electrodes, or the like, and may be positioned at any positionalong the distal portion 25 of lead 16. Further, electrodes 28, 30 and31 may be of similar type, shape, size and material or may differ fromeach other.

Lead 16 extends subcutaneously or submuscularly over the ribcage 32medially from the connector assembly 27 of ICD 14 toward a center of thetorso of patient 12, e.g., toward xiphoid process 20 of patient 12. At alocation near xiphoid process 20, lead 16 bends or turns and extendssuperior subcutaneously or submuscularly over the ribcage and/orsternum, substantially parallel to sternum 22. Although illustrated inFIGS. 1A and 1B as being offset laterally from and extendingsubstantially parallel to sternum 22, lead 16 may be implanted at otherlocations, such as over sternum 22, offset to the right or left ofsternum 22, angled laterally from sternum 22 toward the left or theright, or the like. Alternatively, lead 16 may be placed along othersubcutaneous or submuscular paths. The path of lead 16 may depend on thelocation of ICD 14, the arrangement and position of electrodes carriedby the lead distal portion 25, and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, 30 and 31 locatedalong the distal portion 25 of the lead body 18. Lead body 18 may betubular or cylindrical in shape. In other examples, the distal portion25 (or all of) the elongated lead body 18 may have a flat, ribbon orpaddle shape. The lead body 18 of lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors extend. However, the techniques disclosed herein are notlimited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body 18are each electrically coupled with respective defibrillation electrodes24 and 26 and pace/sense electrodes 28, 30 and 31. Each of pacing andsensing electrodes 28, 30 and 31 are coupled to respective electricalconductors, which may be separate respective conductors within the leadbody. The respective conductors electrically couple the electrodes 24,26, 28, 30 and 31 to circuitry, such as a therapy circuit and/or asensing circuit, of ICD 14 via connections in the connector assembly 17,including associated electrical feedthroughs crossing housing 15. Theelectrical conductors transmit therapy from a therapy circuit within ICD14 to one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28, 30 and 31 and transmit sensed electricalsignals from one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28, 30 and 31 to the sensing circuit within ICD14.

ICD 14 may obtain electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 28, 30, and/or 31. In some examples, housing15 of ICD 14 is used in combination with one or more of electrodes 28,30 and/or 31 in a sensing electrode vector. ICD 14 may even obtaincardiac electrical signals using a sensing vector that includes one orboth defibrillation electrodes 24 and/or 26, e.g., between electrodes 24and 26 or one of electrodes 24 or 26 in combination with one or more ofelectrodes 28, 30, 31, and/or the housing 15.

ICD 14 analyzes the cardiac electrical signals received from one or moreof the sensing vectors to monitor for abnormal rhythms, such asbradycardia, ventricular tachycardia (VT) or ventricular fibrillation(VF). ICD 14 may analyze the heart rate and/or morphology of the cardiacelectrical signals to monitor for tachyarrhythmia in accordance with anyof a number of tachyarrhythmia detection techniques. One exampletechnique for detecting tachyarrhythmia is described in U.S. Pat. No.7,761,150 (Ghanem, et al.), incorporated by reference herein in itsentirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF). ICD 14 may deliver ATPin response to VT detection, and in some cases may deliver ATP prior toa CV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. ATP may be delivered usingan extra-cardiovascular pacing electrode vector selected from any ofelectrodes 24, 26, 28, 30, 31 and/or housing 15. The pacing electrodevector may be different than the sensing electrode vector. In oneexample, cardiac electrical signals are sensed between pace/senseelectrodes 28 and 30, and ATP pulses (or other cardiac pacing pulses)are delivered between pace/sense electrode 30 used as a cathodeelectrode and defibrillation electrode 24 used as a return anodeelectrode. In other examples, cardiac pacing pulses may be deliveredbetween pace/sense electrode 28 and either (or both) defibrillationelectrode 24 or 26 or between defibrillation electrode 24 anddefibrillation electrode 26. These examples are not intended to belimiting, and it is recognized that other sensing electrode vectors andcardiac pacing electrode vectors may be selected according to individualpatient need.

If ATP does not successfully terminate VT or when VF is detected, ICD 14may deliver one or more cardioversion or defibrillation (CV/DF) shocksvia one or both of defibrillation electrodes 24 and 26 and/or housing15. ICD 14 may deliver the CV/DF shocks using electrodes 24 and 26individually or together as a cathode (or anode) and with the housing 15as an anode (or cathode). ICD 14 may generate and deliver other types ofelectrical stimulation pulses such as post-shock pacing pulses orbradycardia pacing pulses using a pacing electrode vector that includesone or more of the electrodes 24, 26, 28, 30 and 31 and the housing 15of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. In otherexamples, lead 16 may include less than three pace/sense electrodes ormore than three pace/sense electrodes and/or a single defibrillationelectrode or more than two electrically isolated or electrically coupleddefibrillation electrodes or electrode segments. The pace/senseelectrodes 28, 30 and/or 31 may be located elsewhere along the length oflead 16. For example, lead 16 may include a single pace/sense electrode30 between defibrillation electrodes 24 and 26 and no pace/senseelectrode distal to defibrillation electrode 26 or proximaldefibrillation electrode 24. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the extra-cardiovascular pacingtechniques disclosed herein are described in U.S. Publication No.2015/0306375 (Marshall, et al.) and U.S. Publication No. 2015/0306410(Marshall, et al.), both of which are incorporated herein by referencein their entirety.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferior from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor52, memory 53, display 54, user interface 56 and telemetry unit 58.Processor 52 controls external device operations and processes data andsignals received from ICD 14. Display 54, which may include a graphicaluser interface, displays data and other information to a user forreviewing ICD operation and programmed parameters as well as cardiacelectrical signals retrieved from ICD 14. For example, as described inconjunction with FIGS. 10 and 11, a clinician may view cardiacelectrical signals received from ICD 14 during VF induction for testingprogrammed sensitivity settings and during normal sinus rhythm forreviewing and selecting programmable R-wave sensing threshold parametersettings.

User interface 56 may include a mouse, touch screen, key pad or the liketo enable a user to interact with external device 40 to initiate atelemetry session with ICD 14 for retrieving data from and/ortransmitting data to ICD 14, including programmable parameters forcontrolling an R-wave sensing threshold as described herein. Telemetryunit 58 includes a transceiver and antenna configured for bidirectionalcommunication with a telemetry circuit included in ICD 14 and isconfigured to operate in conjunction with processor 52 for sending andreceiving data relating to ICD functions via communication link 42.

Communication link 42 may be established between ICD 14 and externaldevice 40 using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi,or Medical Implant Communication Service (MICS) or other RF orcommunication frequency bandwidth. Data stored or acquired by ICD 14,including physiological signals or associated data derived therefrom,results of device diagnostics, and histories of detected rhythm episodesand delivered therapies, may be retrieved from ICD 14 by external device40 following an interrogation command.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may alternatively be embodied as a homemonitor or hand held device. External device 40 may be used to programcardiac signal sensing parameters, cardiac rhythm detection parametersand therapy control parameters used by ICD 14. At least some controlparameters used to control the R-wave sensing threshold according totechniques disclosed herein may be programmed into ICD 14 using externaldevice 40.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement, lead 16 ofsystem 10 is implanted at least partially underneath sternum 22 ofpatient 12. Lead 16 extends subcutaneously or submuscularly from ICD 14toward xiphoid process 20 and at a location near xiphoid process 20bends or turns and extends superiorly within anterior mediastinum 36 ina substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22.In some instances, the anterior wall of anterior mediastinum 36 may alsobe formed by the transversus thoracis muscle and one or more costalcartilages. Anterior mediastinum 36 includes a quantity of looseconnective tissue (such as areolar tissue), adipose tissue, some lymphvessels, lymph glands, substernal musculature, small side branches ofthe internal thoracic artery or vein, and the thymus gland. In oneexample, the distal portion 25 of lead 16 extends along the posteriorside of sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36.

A lead implanted such that the distal portion 25 is substantially withinanterior mediastinum 36 may be referred to as a “substernal lead.” Inthe example illustrated in FIGS. 2A-2C, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. In some instances, lead 16 may extend laterally such that distalportion 25 of lead 16 is underneath/below the ribcage 32 in addition toor instead of sternum 22. In other examples, the distal portion 25 oflead 16 may be implanted in other extra-cardiovascular, intra-thoraciclocations, including the pleural cavity or around the perimeter of andadjacent to but typically not within the pericardium 38 of heart 8.Other implant locations and lead and electrode arrangements that may beused in conjunction with the techniques described herein are generallydisclosed in the above-incorporated patent applications.

FIG. 3 is a conceptual diagram illustrating a distal portion 25′ ofanother example of extra-cardiovascular lead 16 of FIGS. 1A-2C having acurving distal portion 25′ of lead body 18′. Lead body 18′ may be formedhaving a curving, bending, serpentine, or zig-zagging shape along distalportion 25′. In the example shown, defibrillation electrodes 24′ and 26′are carried along curving portions of the lead body 18′. Pace/senseelectrode 30′ is carried in between defibrillation electrodes 24′ and26′. Pace/sense electrode 28′ is carried proximal to the proximaldefibrillation electrode 24′. No electrode is provided distal todefibrillation electrode 26′ in this example.

As shown in FIG. 3, lead body 18′ may be formed having a curving distalportion 25′ that includes two “C” shaped curves, which together mayresemble the Greek letter epsilon, “ε.” Defibrillation electrodes 24′and 26′ are each carried by one of the two respective C-shaped portionsof the lead body distal portion 25′, which extend or curve in the samedirection away from a central axis 33 of lead body 18′. In the exampleshown, pace/sense electrode 28′ is proximal to the C-shaped portioncarrying electrode 24′, and pace/sense electrode 30′ is proximal to theC-shaped portion carrying electrode 26′. Pace/sense electrodes 28′ and30′ may, in some instances, be approximately aligned with the centralaxis 33 of the straight, proximal portion of lead body 18′ such thatmid-points of defibrillation electrodes 24′ and 26′ are laterally offsetfrom electrodes 28′ and 30′. Other examples of extra-cardiovascularleads including one or more defibrillation electrodes and one or morepacing and sensing electrodes carried by curving, serpentine, undulatingor zig-zagging distal portion of the lead body that may be implementedwith the pacing techniques described herein are generally disclosed inU.S. patent application Ser. No. 14/963,303, incorporated herein byreference in its entirety.

FIG. 4 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 4) includes software, firmware and hardware thatcooperatively monitor one or more cardiac electrical signals, determinewhen an electrical stimulation therapy is necessary, and delivertherapies as needed according to programmed therapy delivery algorithmsand control parameters. The software, firmware and hardware areconfigured to detect and discriminate VT and VF for determining when ATPor CV/DF shocks are required and may determine when bradycardia pacingis needed. ICD 14 is coupled to an extra-cardiovascular lead, such aslead 16 carrying extra-cardiovascular electrodes 24, 26, 28, and 30 asshow in FIGS. 3 and 31, if available, as shown in FIG. 1A, fordelivering electrical stimulation pulses to the patient's heart and forsensing cardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. A power source98 provides power to the circuitry of ICD 14, including each of thecomponents 80, 82, 84, 86, and 88 as needed. Power source 98 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 98 andeach of the other components 80, 82, 84, 86 and 88 are to be understoodfrom the general block diagram of FIG. 4, but are not shown for the sakeof clarity. For example, power source 98 may be coupled to a low voltage(LV) charging circuit and to a high voltage (HV) charging circuitincluded in therapy delivery circuit 84 for charging low voltage andhigh voltage capacitors, respectively, included in therapy deliverycircuit 84 for producing respective low voltage pacing pulses, such asbradycardia pacing, post-shock pacing or ATP pulses, or for producinghigh voltage pulses, such as CV/DF shock pulses. In some examples, highvoltage capacitors are charged and utilized for delivering pacing pulsesinstead of low voltage capacitors.

The functional blocks shown in FIG. 4 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include one or more of an application specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, state machine, or othersuitable components or combination of components that provide thedescribed functionality. The particular form of software, hardwareand/or firmware employed to implement the functionality disclosed hereinwill be determined primarily by the particular system architectureemployed in the ICD and by the particular detection and therapy deliverymethodologies employed by the ICD. Providing software, hardware, and/orfirmware to accomplish the described functionality in the context of anymodern ICD system, given the disclosure herein, is within the abilitiesof one of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80 orother ICD components to perform various functions attributed to ICD 14or those ICD components. The non-transitory computer-readable mediastoring the instructions may include any of the media listed above.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as components isintended to highlight different functional aspects and does notnecessarily imply that such components must be realized by separatehardware or software components. Rather, functionality associated withone or more components may be performed by separate hardware, firmwareor software components, or integrated within common hardware, firmwareor software components. For example, sensing operations may be performedby sensing circuit 86 under the control of control circuit 80 and mayinclude operations implemented in a processor executing instructionsstored in memory 82 and control signals such as timing and sensingthreshold amplitude signals sent from control circuit 80 to sensingcircuit 86.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, and 30 (and 31 if presentas shown in FIGS. 1A and 2A) carried by lead 16 and the housing 15,which may function as a common or ground electrode or as an active canelectrode for delivering CV/DF shock pulses or cardiac pacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30 and31 and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector. Sensing circuit 86 is enabled to selectively monitorone or more sensing vectors at a time selected from the availableelectrodes 24, 26, 28, 30, 31 and housing 15. For example, sensingcircuit 86 may include switching circuitry for selecting which ofelectrodes 24, 26, 28, 30, 31 and housing 15 are coupled to senseamplifiers or other cardiac event detection circuitry included insensing circuit 86. Switching circuitry may include a switch array,switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple components of sensing circuit 86 toselected electrodes. In some instances, control circuit 80 may controlthe switching circuitry to selectively couple sensing circuit 86 to oneor more sense electrode vectors. The cardiac event detection circuitrywithin sensing circuit 86 may include one or more sense amplifiers,filters, rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), or other analog or digital components.

In some examples, sensing circuit 86 includes multiple sensing channelsfor acquiring cardiac electrical signals from multiple sensing vectorsselected from electrodes 24, 26, 28, 30, 31 and housing 15. Each sensingchannel may be configured to amplify, filter and rectify the cardiacelectrical signal received from selected electrodes coupled to therespective sensing channel to improve the signal quality for sensingcardiac events, such as R-waves. For example, each sensing channel mayinclude a pre-filter and amplifier for filtering and amplifying a signalreceived from a selected pair of electrodes. The resulting raw cardiacelectrical signal may be passed from the pre-filter and amplifier tocardiac event detection circuitry for sensing cardiac events from thereceived cardiac electrical signal. Cardiac event detection circuitrymay include a rectifier, post-filter and amplifier, a sense amplifier,comparator, and/or analog-to-digital converter for detecting a cardiacevent when the cardiac electrical signal crosses a sensing threshold.The sensing threshold is automatically adjusted by sensing circuit 86under the control of control circuit 80, based on timing intervals andsensing threshold values determined by control circuit 80, stored inmemory 82, and/or controlled by hardware of control circuit 80 and/orsensing circuit 86. Some sensing threshold control parameters may beprogrammed by a user and passed from control circuit 80 to sensingcircuit 86 via a data bus.

As described herein, sensing circuit 86 may sense R-waves according to asensing threshold that is automatically adjusted to multiple thresholdlevels at specified times after a sensing threshold crossing. Multiplethreshold levels and the time intervals over which each threshold levelor value is applied may be used to provide accurate R-wave sensing whileminimizing T-wave oversensing and P-wave oversensing. If T-waves and/orP-waves are falsely sensed as R-waves, due to a cardiac electricalsignal crossing the R-wave sensing threshold, a tachyarrhythmia may befalsely detected potentially leading to an unnecessary cardiacelectrical stimulation therapy, such as ATP or shock delivery. Thissituation is avoided using the multi-level sensing threshold techniquesdisclosed herein while still providing VT and VF detection with a highsensitivity. Oversensing may also cause ICD 14 to inhibit bradycardiapacing pulses when pacing is actually needed. By avoiding oversensingusing the multi-level sensing threshold, inhibiting of bradycardiapacing pulses when pacing is actually needed is avoided.

Upon detecting a cardiac event based on a sensing threshold crossing,sensing circuit 86 may produce a sensed event signal, such as an R-wavesensed event signal, that is passed to control circuit 80. The sensedevent signals are used by control circuit 80 for detecting cardiacrhythms and determining a need for therapy. Sensing circuit 86 may alsopass a digitized electrocardiogram (ECG) signal to control circuit 80for morphology analysis performed for detecting and discriminating heartrhythms. In some examples, analysis of the digitized cardiac electricalsignal is performed for determining R-wave sensing threshold controlparameters as described in conjunction with FIG. 11.

Signals from the selected sensing vector may be passed through abandpass filter and amplifier, provided to a multiplexer and thereafterconverted to multi-bit digital signals by an analog-to-digitalconverter, all included in sensing circuit 86, for storage in randomaccess memory included in memory 82 under control of a direct memoryaccess circuit via a data/address bus. Control circuit 80 may be amicroprocessor based controller that employs digital signal analysistechniques to characterize the digitized signals stored in random accessmemory of memory 82 to recognize and classify the patient's heart rhythmemploying any of numerous signal processing methodologies for analyzingcardiac signals and cardiac event waveforms, e.g., R-waves. Examples ofalgorithms that may be performed by ICD 14 for detecting, discriminatingand treating tachyarrhythmia which may be adapted to utilizemulti-sensing threshold techniques described herein for sensing cardiacelectrical signals are generally disclosed in U.S. Pat. No. 5,354,316(Keimel); U.S. Pat. No. 5,545,186 (Olson, et al.); U.S. Pat. No.6,393,316 (Gillberg et al.); U.S. Pat. No. 7,031,771 (Brown, et al.);U.S. Pat. No. 8,160,684 (Ghanem, et al.), and U.S. Pat. No. 8,437,842(Zhang, et al.), all of which patents are incorporated herein byreference in their entirety.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices, such as one or more high voltage capacitors andin some examples one or more low voltage capacitors, and switchingcircuitry that controls when the capacitor(s) are discharged across aselected pacing electrode vector or CV/DF shock vector. Charging ofcapacitors to a programmed pulse amplitude and discharging of thecapacitors for a programmed pulse width may be performed by therapydelivery circuit 84 according to control signals received from controlcircuit 80. Control circuit 80 may include various timers or countersthat control when ATP or other cardiac pacing pulses are delivered.

For example, control circuit 80 may include pacer timing and controlcircuitry having programmable digital counters set by the microprocessorof the control circuit 80 for controlling the basic time intervalsassociated with various pacing modes or anti-tachycardia pacingsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

During pacing, escape interval counters within the pacer timing andcontrol circuitry are reset upon sensing of R-waves as indicated bysignals from sensing circuit 86. In accordance with the selected mode ofpacing, pacing pulses are generated by a pulse output circuit of therapydelivery circuit 84. The pace output circuit is coupled to the desiredelectrodes via switch matrix for discharging one or more capacitorsacross the pacing load. The escape interval counters are reset upongeneration of pacing pulses, and thereby control the basic timing ofcardiac pacing functions. The durations of the escape intervals aredetermined by control circuit 80 via a data/address bus. The value ofthe count present in the escape interval counters when reset by sensedR-waves can be used to measure R-R intervals for detecting theoccurrence of a variety of arrhythmias.

Memory 82 includes read-only memory (ROM) in which stored programscontrolling the operation of the control circuit 80 reside. Memory 82may further include random access memory (RAM) configured as a number ofrecirculating buffers capable of holding a series of measured intervals,counts or other data for analysis by the control circuit 80 forpredicting or diagnosing an arrhythmia.

In response to the detection of VT, ATP therapy can be delivered byloading a regimen from the microprocessor included in control circuit 80into the pacer timing and control circuit according to the type and rateof tachycardia detected. In the event that higher voltage cardioversionor defibrillation pulses are required, the control circuitmicroprocessor activates cardioversion and defibrillation controlcircuitry included in control circuit 80 to initiate charging of thehigh voltage capacitors of via a charging circuit, both included intherapy delivery circuit 84, under the control of a high voltagecharging control line. The voltage on the high voltage capacitors ismonitored via a voltage capacitor line, which is passed to controlcircuit 80. When the voltage reaches a predetermined value set by themicroprocessor of control circuit 80, a logic signal is generated on acapacitor full line passed to therapy delivery circuit 84, terminatingcharging. The defibrillation or cardioversion pulse is delivered to theheart under the control of the pacer timing and control circuitry by anoutput circuit of therapy delivery circuit 84 via a control bus. Theoutput circuit determines the electrodes used for delivering thecardioversion or defibrillation pulse and the pulse wave shape. Therapydelivery and control circuitry generally disclosed in any of theabove-incorporated patents may be implemented in ICD 14.

Control parameters utilized by control circuit 80 for detecting cardiacrhythms and controlling therapy delivery may be programmed into memory82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication as described above. Under the control of controlcircuit 80, telemetry circuit 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrycircuit 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

FIG. 5 is a timing diagram 100 showing a band-pass filtered andrectified cardiac electrical signal 102 and R-wave sensed event signals103 produced by sensing circuit 86. The cardiac electrical signal 102includes R-waves 104, 104′, T-wave 106, and P-wave 108. As used herein,“R-wave sensing” generally refers to sensing the intrinsic QRS complexof a cardiac electrical signal for the purposes of detecting anddiscriminating intrinsic ventricular rhythms, e.g., for detecting anddiscriminating ventricular fibrillation, ventricular tachycardia,supraventricular tachycardia or other types of intrinsic heart rhythms.Sensing circuit 86 automatically adjusts an R-wave sensing threshold 110to multiple threshold values 116, 118, and 120. The multiple thresholdvalues 116, 118, and 120 may be determined by control circuit 80 basedon the maximum peak amplitude 112 of a sensed R-wave 104 and passed tosensing circuit 86 along with multiple timing intervals 130, 132, 134and 136 for controlling the R-wave sensing threshold 110 for detectingof the next R-wave 104′. Threshold values 116, 118 and 120 of R-wavesensing threshold 110 may also be referred to as threshold “levels” or“settings” or merely as “thresholds” but all refer to different voltageamplitudes which, when crossed by a positive-going, rectified band-passfiltered cardiac electrical signal, result in an R-wave sensed eventsignal being produced by sensing circuit 86.

In the example shown, R-wave 104 is sensed by sensing circuit 86 whenthe cardiac electrical signal 102 crosses R-wave sensing threshold 110,which is set to threshold value 120 at the time of the thresholdcrossing. An R-wave sense event signal 150 is generated. Upon sensingthe R-wave 104, a blanking interval 130 may be started. The blankinginterval may be a fixed time interval controlled by hardware thatprevents the R-wave 104 from being sensed twice. The blanking interval130 is 120 ms in one example, and may be 120 ms to 160 ms in otherexamples. During the blanking interval 130, a peak detector circuitincluded in sensing circuit 86 or control circuit 80 determines themaximum peak amplitude 112 of R-wave 104.

At the expiration of the blanking interval 130, the maximum peakamplitude 112 is used to determine the sensing threshold values 116 and118. In one example, a second blanking interval 132 is started uponsensing R-wave 104 and is slightly longer than the first blankinginterval 130. Upon expiration of the first blanking interval 130, amicroprocessor within control circuit 80 may fetch the R-wave peakamplitude and determine the first, starting threshold value 116 prior toexpiration of the second blanking interval 132. The R-wave sensingthreshold 110 may be set to the starting threshold value 116 uponexpiration of the second blanking interval 132.

The second blanking interval 132 may be at least 20 ms longer than thefirst blanking interval 130. For example, second blanking interval 132may be 140 ms to 180 ms long in some examples. In some implementations,the first blanking interval 130 is a hardware controlled blankinginterval, and the second blanking interval 132 is a digital blankinginterval controlled by firmware or software stored in memory 82. Thesecond blanking interval 132 may be a user-programmable value so that itmay be tailored to the patient, e.g., based on the width of R-wave 104.The control circuit 80 may determine the first, starting threshold value116 as a percentage of the R-wave peak amplitude 112 during the intervalbetween the expiration of the first blanking interval 130 and theexpiration of the second blanking interval 132. This interval differencebetween the first and second blanking intervals 130 and 132 may beminimized in some examples to enable firmware processing time todetermine the first, starting threshold value 116 but enable R-wavesensing as early as possible after expiration of the first blankinginterval 130.

By implementing the second blanking interval 132 and computation of thestarting threshold value 116 in firmware stored in memory 82 andexecuted by a microprocessor of control circuit 80, the multi-levelR-wave sensing threshold 110 disclosed herein may be implemented in manyexisting ICD systems that already include a hardware-implementedblanking interval without requiring hardware modifications. The longersecond blanking interval 132 may be fixed or programmable to account forwider R-waves that typically appear in a cardiac signal obtained usingextra-cardiovascular electrodes compared to the R-wave width inintracardiac electrogram signals. Furthermore, implementation of thesecond blanking interval 132 as a digital blanking interval allowsR-wave sensing threshold control techniques disclosed herein to operatein conjunction with other algorithms or methods being executed byhardware or firmware of ICD 14 for heart rhythm detection withoutmodification. For example, ICD 14 may be configured to execute T-waveoversensing rejection algorithms implemented in hardware or firmwareconfigured to determine a differential filtered cardiac electricalsignal as generally disclosed in U.S. Pat. No. 7,831,304 (Cao, et al.),incorporated herein by reference in its entirety. By setting the secondblanking interval 132 as a digital blanking interval controlled byfirmware, the R-wave sensing threshold 110 may be controlled withoutaltering operations performed by a T-wave oversensing rejectionalgorithm operating concurrently, which may be implemented in hardware.

In other examples, the first blanking interval 130, peak detector fordetermining R-wave peak amplitude 112, and the second blanking interval132 may all be implemented in hardware, all be implemented in firmwareor a combination of both. In some examples, the second blanking interval132 may be programmable such that the time of the onset of R-wavesensing at the expiration of the second blanking interval 132, after theexpiration of the first blanking interval 130, may be selected by a useraccording to patient need. When peak detection and determination of thestarting threshold value 116 are implemented in hardware, a singlehardware implemented blanking interval 130 may be used without requiringa second blanking interval 132 for providing firmware processing timeduring which the starting threshold value 116 is determined.

The first sensing threshold value 116 may set to a percentage of theR-wave peak amplitude 112. For example, the first sensing thresholdvalue 116 may be 50% of peak amplitude 112, and may be from 40% to 60%in other examples. The percentage of peak amplitude 112 used todetermine the starting threshold value 116 is selected to promote a highlikelihood that the threshold value 116 is greater than a maximumamplitude of T-wave 106. The percentage of peak amplitude 112 used todetermine starting threshold value 116 may be selected based on aprevious baseline T-wave amplitude measurement or T/R amplitude ratio.The starting threshold value 116 is held constant over a sense delayinterval 134 in the example shown to maintain the R-wave sensingthreshold 110 above a maximum T-wave amplitude until a time point nearthe end or after the T-wave 106. In other examples, the R-wave sensingthreshold 110 may have a starting threshold value 116 that slowly decaysover the sense delay interval 134. The decay rate, however, is selectedto be relatively slow so that the ending threshold value at theexpiration of the sense delay interval 134 is still greater than anexpected T-wave amplitude.

Sense delay interval 134 may be started upon sensing R-wave 104, asshown in FIG. 5. Alternatively sense delay interval 134 may be startedupon expiration of the second blanking interval 132. Sense delayinterval 134 may be a user-programmable interval which may be tailoredto patient need to encompass the T-wave 106, or at least the peak of theT-wave or a majority of the T-wave 106, to avoid T-wave oversensing.Sense delay interval 134 is 360 ms in one example and may be, with nolimitation intended, 300 ms to 400 ms in other examples. By allowing auser to program the sense delay interval 134, the user has the abilityto make adjustments to how early after a sensed R-wave the sensingthreshold 110 is adjusted to a lower value, e.g., threshold value 118.In this way, if T-wave oversensing is being detected or reported by theICD 14 or is being observed in cardiac electrical signal episodes thatare stored by ICD 14 and transmitted to external device 40, theclinician has the ability to increase the sense delay interval 134 toavoid future T-wave oversensing without compromising detection ofventricular fibrillation or ventricular tachycardia.

Control circuit 80 may be configured to detect T-wave oversensing whenit occurs and reject RR-intervals or other evidence of VT or VF whenT-wave oversensing is detected. Examples of T-wave oversensing rejectionalgorithms that may be included in ICD 14 are generally disclosed in theabove-incorporated '304 patent (Cao, et al.) and in U.S. Pat. No.8,886,296 (Patel, et al.) and U.S. Pat. No. 8,914,106 (Charlton, etal.), also incorporated herein by reference in their entirety. In someexamples, control circuit 80 may automatically increase the sense delayinterval 134 and/or increase the starting value 116 of R-wave sensingthreshold 110 in response to T-wave oversensing detection. Sense delayinterval 134 may be increased up to a predefined maximum limit, e.g.,440 ms. If there is no TWOS detected for a predetermined time interval,for example one minute, one hour or one day, or if a tachyarrhythmiaepisode is being detected (e.g., three or more VT or VF intervalsdetected), sense delay interval 134 may be automatically reduced to ashorter interval or to a previous setting by control circuit 80.

In some examples, sense delay interval 134 is set equal to thetachycardia detection interval (TDI) used by control circuit 80 fordetecting ventricular tachycardia (VT). Alternatively, sense delayinterval 134 may be set slightly longer than the TDI, e.g., 10 to 20 mslonger than the TDI. Intervals between consecutively sensed R-waves, forexample RR interval 140 between two consecutive R-wave sensed eventsignals 150 and 152 shown in FIG. 5, are compared to the TDI and to afibrillation detection interval (FDI) by a cardiac rhythm analyzerincluded in control circuit 80. If an RR interval is less than the TDI,the cardiac rhythm analyzer may increase a VT interval counter. If theRR interval is less than the FDI, the cardiac rhythm analyzer mayincrease a VF interval counter. If the VT counter reaches a number ofintervals to detect (NID) VT, VT is detected. If the VF counter reachesan NID to detect VF, VF is detected. By setting sense delay interval 134to equal a programmed TDI, the R-wave sensing threshold 110 is kepthigh, at the starting threshold value 116, throughout the FDI and theTDI (which is longer than the FDI) such that the likelihood of a falselysensed R-wave due to T-wave oversensing during the TDI is minimized,minimizing the likelihood an oversensed T-wave contributing to a VT orVF detection. If the T-wave 106 exceeds a lower value of R-wave sensingthreshold 110 at an interval after the R-wave 104 that is longer thanthe TDI, the T-wave oversensed event will not contribute to VTdetection. Accordingly, the sense delay interval 134 may be set to matchthe TDI programmed for VT detection and may be automatically adjusted totrack the TDI if the TDI is reprogrammed to a different value.

Upon expiration of the sense delay interval 134, the sensing circuit 86adjusts R-wave sensing threshold 110 to a second threshold value 118,lower than the starting value 116. The second threshold value 118 may bedetermined as a percentage of the R-wave peak amplitude 112. In oneexample, threshold value 118 is set to approximately 28% of the R-wavepeak amplitude 112. Threshold value 118 may be set to 20% to 30% of theR-wave peak amplitude 112 in other examples. The second threshold value118 is set to a value that is expected to be greater than the peakamplitude of the P-wave 108. P-waves are generally much lower inamplitude than R-waves, however, depending on the alignment of thesensing electrode vector relative to the cardiac axis and other factors,P-wave oversensing can occur in some patients, particularly when thelead 16 is positioned substernally as shown in FIG. 2A.

R-wave sensing threshold 110 is held at the second threshold value 118until the expiration of a drop time interval 136. Drop time interval 136may be started at the time R-wave 104 is sensed, as shown in FIG. 5, orupon expiration of blanking interval 130, blanking interval 132, orsense delay interval 134. When drop time interval 136 is started uponsensing R-wave 104, it may be set to 1.5 seconds or other relativelylong interval to promote a high likelihood of maintaining the R-wavesensing threshold at the second value 118 until after P-wave 108, or atleast until after the peak amplitude or majority of P-wave 108. Droptime interval 136 may be a fixed interval or may be programmable by theuser.

The drop time interval 136 may range from 0.8 to 2.0 seconds in otherexamples. In some examples, the drop time interval 136 may be adjustedwith changes in heart rate. For example, as heart rate increases basedon RR interval 140 measurements, the drop time interval 136 may beshortened. As heart rate decreases, the drop time interval 136 may beincreased.

The second threshold value 118 is shown to be a constant value from theexpiration of sense delay interval 134 until the expiration of drop timeinterval 136. In other examples, the second threshold value 188 mayslowly decay until the expiration of drop time interval 136. The decayrate would be selected to be slow, however, so that the ending R-wavesensing threshold at the expiration of the drop time interval 136 isstill expected to be greater than the P-wave amplitude to avoid P-waveoversensing. An example decay rate might be 10% of the maximum peakamplitude 112 per second.

If the cardiac electrical signal has not crossed R-wave sensingthreshold 110 prior to expiration of the drop time interval 136, theR-wave sensing threshold 110 is adjusted from the second sensingthreshold value 118 to a minimum sensing threshold value 120, which maybe referred to as the “sensing floor.” The R-wave sensing threshold 110remains at the minimum sensing threshold 120 until the cardiacelectrical signal 102 crosses the threshold 120. In the example shown,R-wave 104′ is sensed when the minimum sensing threshold 120 is crossed,causing sensing circuit 86 to generate R-wave sense event signal 152.

In some examples, the minimum sensing threshold value 120 is set equalto the programmed sensitivity setting 122 which may be, for example,0.07 millivolts (mV), 0.15 mV, 0.3 mV, 0.6 mV or higher. The programmedsensitivity setting 122 may establish the minimum possible sensingthreshold value in some examples; in which case the R-wave sensingthreshold 110 is never set below the programmed sensitivity setting 122.The sensitivity setting 122 may be programmable between 0.075 and 1.2millivolts (mV) in one example and may be selected by a user as theminimum voltage threshold required to sense a cardiac event from cardiacsignal 102. As the value of the sensitivity setting 122 decreases,sensitivity of the sensing circuit for sensing low amplitude signalsincreases. As such, a low sensitivity setting 122 corresponds to highsensitivity for sensing R-waves. The lowest setting, e.g., 0.07 mV,corresponds to the highest sensitivity, and the highest setting, e.g.,1.2 mV, corresponds to the lowest sensitivity of sensing circuit 86 forsensing R-waves.

Peaks of the cardiac electrical signal 102 that have a maximum voltagebelow the programmed sensitivity setting 122 are considered noise orevents that are not intended to be sensed, which may include P-waves insome examples. When T-wave or P-wave sensing is detected or observed,the user may reprogram the sensitivity setting 122 to a higher setting(lower sensitivity). However, by providing the multi-threshold R-wavesensing threshold 110, controlled using a programmable sense delay timeinterval 134 and the drop time interval 136, the programmed sensitivitysetting 122 may be kept at a low value to provide high sensitivity forsensing R-waves and low amplitude fibrillation waves while stillminimizing the likelihood of T-wave and P-wave oversensing.

In addition to determining the starting threshold value 116 and thesecond threshold value 118, control circuit 80 may establish a maximumR-wave sensing threshold limit 114 that limits the maximum startingvalue of R-wave sensing threshold 110. In some cases, a maximum R-wavesensing threshold limit 114 is set as a fixed multiple or fixed gain ofthe programmed sensitivity setting 122, for example a gain of eight toten times the sensitivity setting 122. In other examples, the gainapplied to the programmed sensitivity setting 122 for establishing amaximum R-wave sensing threshold limit 114 is a variable gain. Thevariable gain may be defined to be dependent on the programmedsensitivity setting 122.

FIG. 6 is a diagram of a filtered and rectified cardiac electricalsignal 200 including R-wave 202 and T-wave 204. In this example, amaximum R-wave sensing threshold limit 216 or 218 is set as a fixedmultiple of the programmed sensitivity setting 220 or 222, respectively.As can be seen in this example, in some cases, when large amplitudeR-waves and T-waves occur, the maximum R-wave sensing threshold limit218 set as a fixed multiple of the programmed sensitivity setting 222may result in T-wave oversensing because T-wave 204 crosses the maximumR-wave sensing threshold limit 218. In this example, the maximum peakamplitude 212 of R-wave 202 is 10 mV, and the sensitivity setting 222 isprogrammed to 0.3 mV. The maximum R-wave sensing threshold limit 218 isset to 3 mV, when a fixed gain of 10 times the sensitivity setting isused to set the maximum threshold limit 218. In this situation of a verylarge R-wave 202, the first sensing threshold value 214 determined as apercentage (50% in the example shown) of the maximum peak amplitude 212of the R-wave 202 is greater than the maximum sensing threshold limit218. As such, the R-wave sensing threshold is set to the maximum sensingthreshold limit 218 at the expiration of the second blanking interval132 until the expiration of sense delay interval 134, which would resultin T-wave oversensing since the maximum limit 218 is less than theamplitude of T-wave 204.

In order to prevent T-wave oversensing, a higher sensitivity setting 220could be programmed, for example 0.6 mV. The maximum sensing thresholdlimit 216 is 6 mV in the example of the programmed sensitivity setting220 being 0.6 mV and a fixed gain of 10 being used to determine themaximum limit 216. This maximum threshold limit 216 is greater than thestarting sensing threshold value 214 determined as a percentage ofR-wave amplitude peak 212, which is 50% of 10 mV or 5 mV in thisexample. This starting threshold value 214 is applied as the R-wavesensing threshold upon expiration of the second blanking interval 132since it is less than the maximum threshold limit 216. The startingthreshold value 214 does not result in T-wave oversensing because theamplitude of T-wave 204 is less than the starting sensing thresholdvalue 214.

As can be seen by the illustrative example of FIG. 6, in the presence oflarge amplitude R-waves and T-waves, T-wave oversensing can occur when amaximum sensing threshold limit is determined as a fixed gain of thesensitivity setting and the sensitivity setting is low. In order toavoid T-wave oversensing, the sensitivity setting can be increased tolower the sensitivity, e.g., to 0.6 mV from 0.3 mV as represented bysensitivity setting 220 and sensitivity setting 222, respectively, inFIG. 6. The higher sensitivity setting, however, makes sensing circuit86 less sensitive to low amplitude R-waves that may occur during VT orVF, potentially resulting in under-detection of ventriculartachyarrhythmia episodes.

FIG. 7 is a diagram of the cardiac electrical signal 200 shown in FIG. 6with maximum sensing threshold limits 256 and 258 determined based on avariable, sensitivity-dependent gain. The gain or multiple of theprogrammed sensitivity setting used by control circuit 80 to determinethe maximum sensing threshold limit is a function of the programmedsensitivity setting in some examples. The maximum sensing thresholdlimit may be inversely related to the programmed sensitivity settingsuch that a higher gain corresponds to a lower programmed sensitivitysetting. For instance, control circuit 80 may compute a variable gainfor determining a maximum sensing threshold limit by determining aninverse proportion of the sensitivity setting and adding a constantusing the equation G=A+B/S where A and B are constants and S is theprogrammed sensitivity (B/S being an inverse proportion of theprogrammed sensitivity setting). In some examples, the gain determinedfor each available programmable sensitivity setting is stored in alook-up table in memory 80 and is retrieved by control circuit 80 eachtime a new sensitivity setting is programmed.

In one example, A is at least 5 and B is at least 2. For instance, A maybe equal to 6 and B may be equal to 2.5 in the equation given for thegain G above. The minimum possible value of the maximum sensingthreshold limit will approach 2.5 since the maximum sensing thresholdlimit is the product of the gain and the programmed sensitivity setting,or 6 S+2.5 where S equals the programmed sensitivity setting. For aprogrammable range of sensitivity settings from 0.075 mV to 1.2 mV, thesensitivity-dependent gain ranges from approximately 39.3 for the lowestsensitivity setting of 0.075 mV (corresponding to highest sensitivity)to approximately 8.1 for the highest sensitivity setting of 1.2 mV(corresponding to the lowest sensitivity). The higher the sensitivity,i.e., the lower the sensitivity setting, the higher thesensitivity-dependent gain is. For a programmed sensitivity of 0.3 mV,G=6+2.5/0.3 or approximately 14.3 in this example. The maximum sensingthreshold limit 258 determined when the sensitivity setting 222 isprogrammed to 0.3 mV is the gain, 14.3, multiplied by the sensitivitysetting, 0.3 mV, or approximately 4.3 mV. This maximum sensing thresholdlimit 258 is less than the first sensing threshold value 254 determinedas a percentage (50% in this example) of R-wave peak amplitude 212. As aresult, the R-wave sensing threshold 210 will be set to the maximumsensing threshold limit 258, but in this case the sensing thresholdlimit 258 set using the variable gain is greater than the amplitude ofT-wave 204, thereby avoiding T-wave oversensing while still allowing ahigh sensitivity (low sensitivity setting) to be used for sensing lowamplitude waveforms during VT or VF (especially spontaneous fine VF).

Continuing with the illustrative example given above, the maximumsensing threshold limit 256 determined for a programmed sensitivitysetting of 0.6 mV 220 is determined using a sensitivity-dependent gainof approximately 10.2 (G=6+2.5/0.6). The maximum sensing threshold limit256 is 6.1 mV in this case (0.6 mV multiplied by the gain of 10.2),which is greater than the starting sensing threshold value 254determined as a percentage (e.g., 50%) of the R-wave peak amplitude 212.In this case, the R-wave sensing threshold 210 is set to the startingsensing threshold value 254 at the expiration of the second blankingperiod 132 (shown in FIG. 5).

In both cases of 0.6 mV sensitivity setting 220 and 0.3 mV sensitivitysetting 222, the R-wave sensing threshold value during the sense delayinterval 134 avoids T-wave oversensing in the presence of largeamplitude R-waves and T-waves. Even when a low sensitivity setting isused, e.g., 0.3 mV or less, so that sensing circuit 86 remains highlysensitive to small R-waves that may occur during a ventriculartachyarrhythmia, T-wave oversensing is avoided by using asensitivity-dependent variable gain for determining the maximum R-wavesensing threshold limit.

At the expiration of the sense delay interval 134, the R-wave sensingthreshold is adjusted from the starting threshold 254 or 258, to thesecond threshold 260 which is determined as 25% of the R-wave peakamplitude 212 in this example. The second threshold 260 remains ineffect until the drop time interval 136 expires (described in FIG. 5)after which the R-wave sensing threshold 210 drops to the programmedsensitivity setting, either the 0.6 mV sensitivity setting 220 or the0.3 mV sensitivity setting 222 in the example shown in FIG. 7.

FIG. 8A is a diagram of a filtered and rectified cardiac electricalsignal 300 including an R-wave 302, a T-wave 304, and a P-wave 306 andan automatically adjusted R-wave sensing threshold 310 having multiplesensing threshold values 316, 317 and 318. In the examples of FIGS. 5, 6and 7, the R-wave sensing threshold 310 is set to the first, startingthreshold value and second threshold value before dropping to theprogrammed sensitivity setting. In other examples, the R-wave sensingthreshold 310 may be adjusted to three or more threshold values beforedropping to the programmed sensitivity setting.

As shown in FIG. 8A, the starting threshold value 316 may be determinedas a first percentage of the R-wave peak amplitude 312 determined duringfirst blanking interval 330, e.g., 62.5% or between 55% and 70% of thepeak R-wave amplitude 312. The starting threshold value 316 may bemaintained from the expiration of the second blanking interval 332 untilthe expiration of a first sense delay interval 333. The first sensedelay interval 333 may be approximately 180 ms, for example 30 to 60 mslonger than the second blanking interval 332. The higher startingthreshold value 316 applied for a short interval may reduce thelikelihood of double sensing the R-wave 302, particularly in patientsexhibiting a wide QRS complex.

Upon expiration of the first sense delay interval 333, the R-wavesensing threshold 310 is adjusted to a lower, second sensing thresholdvalue 317, which may be between 30% and 60% of the R-wave peak amplitude312, such as 50% of the R-wave peak amplitude 312. The second sensingthreshold value 317 is maintained until expiration of the second sensedelay interval 334, which may be between 300 and 360 ms, and may be setequal to a programmed TDI as described previously in conjunction withFIG. 5.

Upon expiration of the second sense delay interval 334, the thirdsensing threshold value 318 is applied until a drop time interval 336expires, and the R-wave sensing threshold 310 falls to a minimum sensingthreshold value 320, which may be equal to the programmed sensitivitysetting. The third sensing threshold value 318 may be approximately 28%of the R-wave peak amplitude 312, or between 20% and 30% in otherexamples, and extend for a drop time interval 336 of one to two seconds,e.g., 1.5 seconds.

FIG. 8B is a diagram of a non-monotonic, multi-level R-wave sensingthreshold 350 according to another example. In the examples of FIGS. 5and 8A, R-wave sensing threshold 110 and R-wave sensing threshold 310,respectively, are monotonically decreasing sensing thresholds. In otherexamples, the multi-level R-wave sensing threshold controlled by controlcircuit 80 is non-monotonic, including one or more step increases inR-wave sensing threshold value in addition to the decreasing stepchanges in sensing threshold value.

The filtered and rectified cardiac electrical signal 300, includingR-wave 302, T-wave 304, and P-wave 306, and an automatically adjustedR-wave sensing threshold 350 are shown in FIG. 8B. R-wave sensingthreshold 350 may include a starting sensing threshold value 316beginning upon expiration of second blanking interval 332 and a secondsensing threshold value 318 beginning after expiration of the sensedelay interval 334. R-wave sensing threshold 350 drops to the programmedsensitivity setting 320 upon expiration of drop time interval 336.

If cardiac signal 300 does not cross the R-wave sensing threshold 350before a maximum sensitivity interval 338 expires, the R-wave sensingthreshold 350 is increased to a third sensing threshold value 322. Thethird sensing threshold value may be equal to the second sensingthreshold value 318 or set as a percentage of a previously determinedbaseline P-wave maximum peak amplitude, e.g., 1.5 times a previouslydetermined P-wave maximum peak amplitude. The maximum sensitivityinterval 338 controls how long the R-wave sensitivity threshold 350 isheld at the maximum sensitivity, i.e., the programmed sensitivitysetting 320, before being increased to the third sensing threshold value322. In some examples, the maximum sensitivity interval 338 isapproximately 200 ms longer than the drop time interval 336 so that theR-wave sensing threshold 350 is set to the programmed sensitivitysetting 320 for up to 200 ms if an R-wave sensing threshold crossingdoes not occur.

Upon expiration of the maximum sensitivity interval 338, R-wave sensingthreshold 350 is increased to the third sensing threshold value 322 tominimize the likelihood of oversensing the P-wave 306 during very slowheart rates, when the P-wave 306 has an amplitude greater than theprogrammed sensitivity setting 320. By allowing the R-wave sensingthreshold 350 to drop to the programmed sensitivity setting 320, toprovide high sensitivity for up to a predefined time interval ascontrolled by interval 338, undersensing of low amplitude, fine VFwaveforms is avoided. Sensing circuit 86 may sense low amplitudeventricular tachyarrhythmia waveforms after expiration of drop timeinterval 336 and before expiration of maximum sensitivity interval 338.If the heart rate is very slow, however, such that the P-wave 306arrives relatively late after R-wave 302 and after expiration ofdrop-time interval 336, P-wave oversensing may be avoided by increasingthe R-wave sensing threshold 350 to the third threshold value 322 whilestill providing an interval of high sensitivity to low amplitudetachyarrhythmia waveforms. The third sensing threshold value 322 may bemaintained until a sensing threshold crossing occurs. In other examples,as shown in FIG. 8B, the third sensing threshold value 322 is held untila second drop time interval 340 expires, at which time the sensingthreshold 350 is adjusted back to the programmed sensitivity setting320.

FIG. 9 is a flow chart 400 of a method for controlling the R-wavesensing threshold according to one example. At block 402, the controlcircuit 80 establishes the maximum threshold limit. The maximumthreshold limit may be set based on a sensitivity-dependent gain asdescribed in conjunction with FIG. 7. Control circuit 80 determines thesensitivity-dependent gain then computes the maximum threshold limit asthe product of the gain and the programmed sensitivity setting.Alternatively, the maximum threshold limit may be set as a fixedmultiple of the programmed sensitivity setting, as described inconjunction with FIG. 6. The maximum threshold limit is re-establishedat block 402 each time the sensitivity is reprogrammed to a differentsensitivity setting.

At block 404, an R-wave is sensed in response to the cardiac electricalsignal crossing the R-wave sensing threshold, which may initially be setto the maximum sensing threshold, a nominal sensing threshold, theprogrammed sensitivity setting or other starting value. In response tosensing an R-wave, sensing circuit 86 produces an R-wave sensed eventsignal at block 408, and control circuit 80 sets various timers orcounters as described in conjunction with FIGS. 5 and 8A and 8B forcontrolling the multi-threshold R-wave sensing threshold. For example, afirst blanking interval, which may be a hardware controlled blankinginterval, a second blanking interval, which may be a digital blankinginterval controlled by firmware stored in memory 82, a sense delayinterval, a drop time interval, and a maximum sensitivity interval maybe started upon sensing an R-wave due to a positive-going R-wave sensingthreshold crossing of the cardiac electrical signal received by sensingcircuit 86.

At block 408, the maximum peak amplitude of the sensed R-wave isdetermined. The R-wave peak amplitude may be determined by a peak trackand hold circuit or other hardware or firmware. The R-wave peakamplitude is fetched by control circuit 80 at the expiration of thefirst blanking interval. Control circuit 80 determines the starting andsecond threshold values at block 410 as two different percentages of theR-wave peak amplitude. For example, the starting threshold value may bedetermined as 40 to 60% of the R-wave peak amplitude and the secondthreshold value may be determined as 20 to 30% of the R-wave peakamplitude. Control circuit 80 may execute firmware after expiration ofthe first blanking interval for determining these starting, firstthreshold value and second threshold value before expiration of thesecond blanking interval. The staring and second threshold values may bepassed to sensing circuit 86 as control values used by circuitry ofsensing circuit 86 for controlling the R-wave sensing threshold. Inother examples, three or more threshold values are determined asdescribed in conjunction with FIGS. 8A and 8B.

Upon expiration of the second blanking interval, as determined at block412, the sensing circuit 86 sets the starting R-wave sensing thresholdto the starting threshold value determined as a percentage of the R-wavepeak amplitude or to the maximum threshold limit, whichever is less,under the control of control circuit 80. If the cardiac electricalsignal crosses the starting R-wave sensing threshold, as determined atblock 416, the process returns to block 406 where sensing circuit 86produces another R-wave sensed event signal and restarts the variouscontrol time intervals as described above.

If the sense delay interval expires at block 418 before the cardiacelectrical signal crosses the R-wave sensing threshold, sensing circuit86 adjusts the R-wave sensing threshold at block 420 to the secondthreshold value received from control circuit 80. If the cardiacelectrical signal crosses the R-wave sensing threshold adjusted to thesecond threshold value, as determined at block 422, the process returnsto block 406 to generate an R-wave sensed event signal and reset thecontrol time intervals as described above. If the drop time intervalexpires at block 424 without the cardiac electrical signal crossing theR-wave sensing threshold (block 422), the sensing circuit 86 adjusts theR-wave sensing threshold to the minimum threshold value or sensingfloor, which may be the programmed sensitivity setting, at block 426.Sensing circuit 86 waits for the cardiac electrical signal to cross thesensing floor at block 404 and the process repeats. In other examples,more than two drop steps in the sensing threshold value may beimplemented, as described in FIG. 8A, and/or a step increase in thesensing threshold value may be included as described in FIG. 8B.

While not shown in FIG. 9, it is recognized that if a pacing therapy isenabled and a pacing pulse is delivered during the sense delay interval(e.g., an ATP pulse) or the drop time interval (e.g., a bradycardia orasystole back up pacing pulse) or any time prior to an R-wave sensingthreshold crossing, the various timers used to control the R-wavesensing threshold may be reset and a previous starting R-wave sensingthreshold value or a starting threshold value based on an average dailyR-wave amplitude may be started following a post-pace blanking interval,which may be a longer blanking interval than the blanking interval usedduring intrinsic R-wave sensing. As such, the various timing intervalsand threshold values shown in any of FIGS. 5-8B may be determined andapplied for controlling the R-wave sensing threshold following adelivered pacing pulse and are not limited to being used following onlyintrinsic R-wave sensed events.

Furthermore, while the techniques have been described for controlling anR-wave sensing threshold for sensing R-waves attendant to ventriculardepolarization, it is to be understood that aspects of the disclosedtechniques may be used for controlling a cardiac event sensing thresholdfor sensing other cardiac signals, such as P-waves attendant to atrialdepolarization or T-waves attendant to ventricular repolarization. Forexample, a maximum P-wave sensing threshold limit may be set based on asensitivity-dependent gain and a programmed sensitivity; a maximumT-wave sensing threshold limit may be set based on asensitivity-dependent gain and programmed sensitivity. P-wave and/orT-wave sensing thresholds may be controlled using multiple thresholdlevels and multiple time intervals.

FIG. 10 is a flow chart of a method for selecting a sensitivity settingin ICD 14. The process shown in FIG. 10 may be performed at the time ofICD 14 implant or during a lead replacement procedure to determine areliable sensitivity setting for detecting low amplitude fibrillationwaves during VF. The process shown by flow chart 500 may be asemi-automated process executed by ICD 14 in response to programmingcommands received from external device 40.

At block 502, a test sensitivity setting is selected. The testsensitivity setting may be programmed by a user using external device 40or may be a value two times a nominal sensitivity setting or a preferredsensitivity setting, which may be based on a measured R-wave amplitude.For example, if the R-wave amplitude observed on an electrocardiogramsignal produced by ICD 14 and transmitted to external device 40 is atleast 3 mV, a test sensitivity setting of 0.6 mV may be set at block 502for a preferred sensitivity setting of 0.3 mV, half of the test setting.

At block 504, a user transmits a VF induction command to ICD 14 usingexternal device 40. ICD 14 may induce VF using any implemented inductionmethod, such as a T-shock, which is a large energy electrical pulsedelivered during the vulnerable period associated with the T-wave. If VFis detected at the programmed sensitivity setting, “Yes” branch of block506, a defibrillation shock is delivered according to programmed shocktherapy control parameters to terminate the VF. If VF is not detected atthe programmed sensitivity within a predetermined time limit, a shock isdelivered to terminate the induced VF, and the sensitivity setting maybe decreased, to increase the sensitivity to VF waveforms, at block 508and VF is induced again. This process may be repeated one or more timesas needed to determine the highest sensitivity setting that allowdetection of VF. Alternatively, if VF is not detected at block 506 usingthe first sensitivity setting, a recording of the cardiac electricalsignal during the induced VF may be used to determine the voltageamplitude of the VF waveforms and the sensitivity setting may beprogrammed lower than the VF waveform amplitude at block 512. In stillother examples, if sensing circuit 86 includes two sensing channels, thecardiac electrical signal may be sensed using two different testsensitivity settings simultaneously to determine if one or both resultin VF detection.

When VF is detected at the current test sensitivity setting, thesensitivity setting is programmed to one-half to one-third the testsensitivity setting. For example, if the sensitivity setting tested is0.6 mV, the sensitivity setting is programmed to 0.3 mV at block 512. Byusing a sensitivity-dependent gain for setting the maximum R-wavesensing threshold limit, a lower sensitivity setting may be tested andused with confidence in avoiding T-wave and P-wave oversensing whilestill providing high sensitivity for detecting VF, both acutely andchronically after implantation of the ICD system 10. The R-waveamplitude of the cardiac electrical signal received by theextra-cardiovascular electrodes is similar during the acute phase (daysor weeks) after implantation and after chronic implantation (months oryears). Accordingly, the recommended sensitivity setting determined atblock 512 need not change based on time since implant, unliketransvenous ICD systems which may have larger R-wave amplitude acutelyand smaller R-wave amplitude chronically. A two-fold or three foldsensitivity safety margin may be used in the extra-cardiovascular ICDsystem 10 rather than higher safety margins which have been practiced inthe past for transvenous ICD systems, such as a four-fold safety margin.Control of the R-wave sensing threshold as disclosed herein using a two-to three-fold sensitivity safety margin minimizes the risk ofundersensing spontaneous fine VF (usually with small waveformamplitudes) while avoiding T-wave and P-wave oversensing.

FIG. 11 is a flow chart 600 of a method for selecting R-wave sensingthreshold control parameters according to one example. As describedabove in conjunction with FIG. 10, the sensitivity setting, which maydefine the minimum R-wave sensing threshold value, may be based at leastin part on VF detection testing. The starting value of the R-wavesensing threshold may be set on a beat-by-beat basis based on the peakR-wave amplitude (as shown in FIG. 5). The gain applied to thesensitivity for setting the maximum R-wave sensing threshold value maybe a variable gain that is dependent on the programmed sensitivitysetting as described in conjunction with FIG. 7. In addition to theseR-wave sensing threshold control parameters, other R-wave sensingthreshold control parameters may be variable and/or programmable basedon cardiac signal features determined for an individual patient totailor optimal R-wave sensing threshold control for that patient and/orbased on empirical data from a population of patients.

For example, the percentage of the R-wave peak amplitude used to set thestarting threshold value 116, the second blanking interval 132, thesense delay interval 134, the second, lower threshold value 118, and thedrop time interval 136 (all shown in FIG. 5) may all be programmable orvariable values that may be tailored to an individual patient and/orbased on sensitivity performance data obtained from a population ofpatients. Other control parameters such as a second sensing delayinterval 334 as shown in FIG. 8A or a maximum sensitivity interval 338as shown in FIG. 8B may also be programmable and tailored individuallyto a patient.

At block 602, population-based VT/VF detection sensitivity for one ormore individual R-wave sensing threshold control parameter settingsand/or combinations of R-wave sensing threshold control parametersettings may be stored in ICD memory 82 and/or in memory 53 of externaldevice 40. For example, a VT/VF detection sensitivity curve as afunction of the programmed sensitivity setting 122, second blankinginterval 132, drop time interval 136, or other R-wave sensing controlparameters described above may be determined from empirical datagathered from a population of ICD patients.

A plot 700 of an illustrative VT/VF detection sensitivity curve 706 isdepicted in FIG. 12. VT/VF detection sensitivity, expressed as thepercentage of all VT/VF episodes actually detected, is plotted along they-axis 702 as a function of an R-wave sensing threshold controlparameter setting plotted along the x-axis 704. The sensing controlparameter is indicated generically in FIG. 12 but may be the secondblanking interval 132, the percentage used to determining the startingvalue 116 of the R-wave sensing threshold, the sense delay interval 134,the percentage used to determine the second value 118 of the R-wavesensing threshold, the drop time interval 136, the sensitivity setting122 (all shown in FIG. 5) or any of the other R-wave sensing thresholdcontrol parameters described herein.

An alert threshold 710 may be set, below which the VT/VF detectionsensitivity falls below VT/VF detection performance expectations, e.g.,95%. When the control parameter setting has a programmed value greaterthan “X”, the VT/VF detection sensitivity falls below the alertthreshold 710. As described below, stored VT/VF detection sensitivitydata may be used by control circuit 80 (or external device processor 52)to look up an expected VT/VF detection sensitivity for a programmedR-wave sensing threshold control parameter individually or incombination with other parameter values in a multi-parametern-dimensional model of detection sensitivity. If the detectionsensitivity falls below an alert threshold 710 for ICD performanceexpectations, a clinician alert may be generated as described below.

FIG. 13 is a plot 800 of an example VT/VF detection sensitivity curve806 as a function of the programmed sensitivity setting 804. Whenprogrammed sensitivity setting is less than approximately 135microvolts, the VT/VF detection sensitivity is greater than the alertthreshold 810, shown as 95% in this example though other alert thresholdlevels may be selected. In this example, when the programmed sensitivitysetting is 140 microvolts or higher, the VT/VF detection sensitivityfalls below the alert threshold 810, and the ICD system 10 may generatean alert displayed on external device display 54 as described below inresponse to a sensitivity setting greater than 140 microvolts beingselected for programming.

FIGS. 12 and 13 represent VT/VF detection sensitivity as a function of asingle sensing threshold control parameter setting. It is recognizedthat instead of a single parameter function as shown in FIG. 12, VT/VFsensitivity may be modeled in a multi-parameter, n-dimensional modeltaking into account a combination of sensing threshold parameters.Furthermore, it is contemplated that instead of sensitivity or inaddition to sensitivity, VT/VF detection specificity may be modeled forone or more sensing threshold parameters, individually or in amulti-parameter, n-dimensional model. Sensing threshold controlparameters may be determined based on baseline cardiac electrical signalfeatures and selected in order to achieve a targeted specificity and/ortargeted sensitivity.

Returning to FIG. 11, at block 604, a processor included in controlcircuit 80 may determine baseline cardiac signal features. During aconfirmed normal sinus rhythm, for example, one or more of the R-waveamplitude, R-wave width, T-wave amplitude, P-wave amplitude, R-T timeinterval, R-P time interval, T-P time interval, and/or baseline noisemay be determined. The R-T, R-P and T-P time intervals may be determinedas time intervals between the absolute maximum peak amplitude of therespective R-, T- and P-waves or between other predefined fiducialpoints of these waves. The normal sinus rhythm may be confirmed manuallyor based on R-R intervals being greater than a tachyarrhythmia detectioninterval, no cardiac pacing being delivered, and/or an R-wave morphologymatch score greater than a predetermined threshold. The cardiac signalfeatures may be determined by control circuit 80 from a digitized,filtered and rectified cardiac signal received from sensing circuit 86.

Alternatively, cardiac signal features may be determined manually from acardiac electrical signal transmitted to and displayed by externaldevice 40 or determined automatically by external device processor 52from the transmitted cardiac electrical signal. The cardiac signalfeature values may then be used by external device processor 52 fordetermining recommended R-wave sensing threshold control parameters orthe cardiac signal feature values may be programmed into ICD 14, storedin ICD memory 82, and retrieved by a processor included in controlcircuit 80 for use in determining R-wave sensing threshold controlparameters.

With continued reference to FIG. 5, at block 606 of FIG. 11, a processorof control circuit 80 may determine control parameters for setting thestarting value 116 of the R-wave sensing threshold. As described abovein conjunction with FIG. 7, a variable gain applied to the sensitivitysetting for determining a maximum R-wave sensing threshold limit may bedetermined based on the programmed sensitivity setting. The maximumR-wave sensing threshold limit is one control parameter used todetermine the starting value 116. Another control parameter is thepercentage of the maximum peak amplitude 112 of the currently sensedR-wave 104 that is used to determine the starting value 116. Thispercentage may be based on a T/R ratio of the peak T-wave voltageamplitude to the peak R-wave voltage amplitude 112 determined from thefiltered, rectified cardiac electrical signal at block 604. For example,if the T/R ratio is 0.5, the starting R-wave sensing threshold may bedetermined as at least 0.6 or 0.7 of the maximum peak R-wave amplitude112 or a percentage of at least 60% or 70%. If the T/R ratio is 0.3, thepercentage may be set to 50% or some other percentage greater than theT/R ratio.

In other examples, a minimum limit of starting value 116 may be setbased on the T-wave amplitude determined at block 604, e.g., a minimumlimit of the starting value may be determined as a percentage greaterthan the maximum peak T-wave voltage amplitude, e.g., 125% of the peakT-wave voltage amplitude or 150% of the peak T-wave voltage amplitude,or a fixed interval greater than the peak T-wave voltage amplitude,e.g., 0.25 mV or 0.5 mV greater than the peak T-wave voltage amplitude.

At block 608, control circuit 80 may determine one or more controlparameters for use in setting the second threshold value 118 (shown inFIG. 5). The second threshold value 118 may be set as a secondpercentage of the peak R-wave voltage amplitude 112 that is less thanthe percentage used to determine the starting value 116. This secondpercentage may be based on the P/R ratio of the maximum peak P-wavevoltage amplitude to the maximum peak R-wave voltage amplitude 112determined from the filtered, rectified cardiac electrical signal atblock 604. For example, if the P/R ratio is 0.2, the second thresholdvalue 118 may be determined as 0.4, or 40%, of the maximum peak R-waveamplitude. If the P/R ratio is 0.3, the percentage may be set to 50% orsome other percentage greater than the P/R ratio.

In other examples, a minimum limit of the second threshold value 118 maybe set based on the P-wave amplitude determined at block 604. Forexample, a minimum limit of the second threshold value 118 may bedetermined as a percentage of the P-wave amplitude, e.g., 125% of theP-wave amplitude or 150% of the P-wave amplitude, or a fixed amountgreater than the maximum peak P-wave voltage amplitude, e.g., 0.2 mV or0.3 mV, or other fixed amount than the peak P-wave amplitude.

At block 610, the control circuit 80 may set the second blankinginterval 132 based on an R-wave width determined at block 604. Asdescribed previously, the first blanking interval 130 may be a hardwareimplemented blanking interval that is absolute and may define a minimumpossible value of the second blanking interval 132. An R-wave widthmeasurement may be determined at block 604 from a bandpass filteredcardiac electrical signal as the time interval from a fiducial point onthe positive-going portion of the R-wave to a fiducial point on thenegative-going portion of the R-wave, e.g., from the first positivecrossing of a predetermined voltage to the last negative-going crossingof the predetermined voltage or from a maximum +dV/dt to a maximum−dV/dt. The second blanking interval 132 may be set to be at least equalto the determined R-wave width, a pre-determined portion of the R-wavewidth, or a fixed interval greater than or less than the R-wave width.The manner in which the second blanking interval 132 is determined basedon an R-wave width may depend on how the R-wave width is determined.Example methods for determining an R-wave width are generally disclosedin U.S. Pat. No. 8,983,586 (Zhang) and U.S. Pat. No. 5,312,441 (Mader,et al.), both patents incorporated herein by reference in theirentirety.

At block 612, control circuit 80 may determine the sense delay interval134 based on the R-T interval determined at block 604. For example,sense delay interval 134 may be a fixed interval longer than the R-Tinterval, e.g., 20 ms longer than the measured R-T interval, or apredetermined percentage of the R-T interval, e.g., 120% of the R-Tinterval.

The drop time interval 136 may be determined by control circuit 80 atblock 614 based on the R-P interval determined at block 604, e.g., as afixed interval or percentage greater than the R-P interval. Since theR-T and R-P intervals may change with heart rate, the control circuit 80may adjust the sense delay interval 134 and the drop time interval 136based on heart rate (e.g., based on a running average of a predeterminednumber of recent RR intervals) in addition to or alternatively to basingthe values on the measured R-T and R-P intervals.

The programmed sensitivity setting may be determined and set at block615 based on the P-wave amplitude and/or baseline noise determined atblock 604. A baseline noise amplitude may be determined by measuring thepeak cardiac signal amplitude during a baseline window set betweencardiac events, e.g., after the T-wave and before an R-wave. Thesensitivity setting may be determined as the lowest setting that isgreater than the determined baseline noise amplitude or a fixed intervalor percentage greater than the determined baseline noise amplitude.

At block 616, control circuit 80 may be configured to compare the R-wavesensing threshold control parameters to the population-based VT/VFdetection sensitivity data stored at block 602. The R-wave sensingthreshold control parameters may include a combination of automaticallydetermined control parameters set by control circuit 80 as describedabove and/or user-programmed control parameters. Individual controlparameter settings or combinations of control parameter settings may becompared to VT/VF detection sensitivity data to predict the expectedsensitivity for detecting VT and VF when the currently selected R-wavesensing threshold control parameters are utilized.

If any of the control parameters, individually or in combination, resultin an expected VT/VF detection sensitivity that is less than the alertthreshold (e.g., threshold 710 in FIG. 12), an alert condition isdetected at block 618. In response to detecting an alert condition,control circuit 80 may generate an alert notification at block 620 thatis transmitted to external device 40 and displayed on user display 54. Auser may then review the programmed settings and make any adjustmentsneeded to improve the expected VT/VF detection sensitivity or accept theprogrammed settings without adjustments. The programmed settings withany adjustments may be transmitted back to ICD 14 and stored in memory82 at block 622 for use in controlling the R-wave sensing threshold.

In some examples, detection sensitivity data are stored in memory 82 ofICD 14 and the process of flow chart 600 is performed automatically bycontrol circuit 80 for setting the sensing threshold control parameters.A targeted VT/VF detection sensitivity value may be programmed into ICD14 by a user and ICD 14 may determine the sensing threshold controlparameters based on the targeted sensitivity and the baseline cardiacsignal features. This process may be repeated periodically for updatingthe sensing threshold control parameters.

In other examples, the VT/VF detection sensitivity data stored at block602 is stored in memory 53 of external device 40. The operations ofblocks 604 through 618 may be performed by a processor included incontrol circuit 80, by external device processor 52 after receiving acardiac electrical signal episode from ICD 14 via ICD telemetry circuit88 and external device telemetry unit 58, or cooperatively by aprocessor of control circuit 80 and external device processor 52 withsome steps or operations performed by control circuit 80 and someperformed by processor 52. Processor 52 may perform the comparison atblock 616 and generate the display of the alert notification at block620 on display 54 in response to detecting an alert condition at block618. Upon user acceptance of the programmed settings of the R-wavesensing threshold control parameters, after any adjustments made basedon an alert if generated, external device 40 transmits the programmablecontrol parameter settings to ICD 14.

Thus, a method and apparatus for controlling R-wave sensing threshold inan extra-cardiovascular ICD system have been presented in the foregoingdescription with reference to specific embodiments. In other examples,various methods described herein may include steps performed in adifferent order or combination than the illustrative examples shown anddescribed herein. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure and the following claims.

The invention claimed is:
 1. A medical device, comprising: a controlcircuit configured to: determine a starting value of a cardiac eventsensing threshold; determine a second value of the cardiac event sensingthreshold, the second value being less than the starting value; set asense delay interval; and start a blanking interval that expires earlierthan the sense delay interval; detect an expiration of the sense delayinterval; and a sensing circuit coupled to the control circuit andconfigured to: receive a cardiac electrical signal; hold the cardiacevent sensing threshold constant at the starting value for from anexpiration of the blanking interval to the expiration of the sense delayinterval; at the expiration of the sense delay interval, set the cardiacevent sensing threshold to the second value to apply the second value ofthe cardiac event sensing threshold starting from the expiration of thesense delay interval; sense a cardiac event attendant to a myocardialdepolarization in response to the cardiac electrical signal crossing thecardiac event sensing threshold; and generate a sensed event signal inresponse to sensing the cardiac event.
 2. The medical device of claim 1,wherein the control circuit is configured to set the sense delayinterval by: determining, from the cardiac electrical signal, an R-Tinterval between an R-wave and a T-wave; and setting the sense delayinterval to be longer than the R-T interval.
 3. The medical device ofclaim 1, wherein the control circuit is configured to determine thestarting value of the cardiac event sensing threshold by: determining afirst amplitude of a T-wave from the cardiac electrical signal; anddetermining the starting value based on at least the first amplitude ofthe T-wave.
 4. The medical device of claim 3, wherein the controlcircuit is further configured to determine the starting value of thecardiac event sensing threshold by: determining a second amplitude of anR-wave from the cardiac electrical signal; determining a ratio of thefirst amplitude to the second amplitude; and determining the startingvalue of the cardiac event sensing threshold based on the ratio of thefirst amplitude to the second amplitude.
 5. The medical device of claim4, wherein the control circuit is further configured to determine thestarting value of the cardiac event sensing threshold by: determining apercentage that is greater than the ratio of the first amplitude to thesecond amplitude; starting the blanking interval in response to thesensing circuit generating a previous sensed event signal; determining apeak amplitude of the cardiac electrical signal during the blankinginterval; determining the percentage of the peak amplitude; anddetermining the starting value of the cardiac event sensing threshold tobe the percentage of the peak amplitude.
 6. The medical device of claim3, wherein the control circuit is further configured to determine thestarting value of the cardiac event sensing threshold by: determining aminimum sensing threshold limit based on the first amplitude of theT-wave; setting the starting value of the cardiac event sensingthreshold to be greater than the minimum sensing threshold limit.
 7. Themedical device of claim 1, wherein the control circuit is furtherconfigured to determine the second value of the cardiac event sensingthreshold by: determining a first amplitude of an R-wave from thecardiac electrical signal; determining a second amplitude of a P-wavefrom the cardiac electrical signal; determining a ratio of the secondamplitude to the first amplitude; and determining the second value ofthe cardiac event sensing threshold based on the ratio of the secondamplitude to the first amplitude.
 8. The medical device of claim 7,wherein the control circuit is further configured to determine thesecond value of the cardiac event sensing threshold by: determining apercentage that is greater than the ratio of the second amplitude to thefirst amplitude; starting the blanking interval in response to thesensing circuit generating a previous sensed event signal; determining apeak amplitude of the cardiac electrical signal during the blankinginterval; determining the percentage of the peak amplitude; anddetermining the second value of the cardiac event sensing threshold tobe the percentage of the peak amplitude.
 9. The medical device of claim1, wherein: the control circuit is further configured to: start theblanking interval in response to a previous sensed event signalgenerated by the sensing circuit; determine the starting value of thecardiac event sensing threshold during the blanking interval; and detectan expiration of the blanking interval; and the sensing circuit isconfigured to set the cardiac event sensing threshold to the startingvalue in response to the expiration of the blanking interval.
 10. Themedical device of claim 1, further comprising a therapy delivery circuitcoupled to the control circuit, wherein: the control circuit is furtherconfigured to: start a pacing escape interval in response to the sensedevent signal generated by the sensing circuit; detect an expiration ofthe pacing escape interval; and the therapy delivery circuit isconfigured to generate a pacing pulse in response to the expiration ofthe pacing escape interval.
 11. The medical device of claim 1, furthercomprising a therapy delivery circuit coupled to the control circuit,wherein: the control circuit is further configured to: determine asensed event interval in response to the sensed event signal generatedby the sensing circuit; determine that the sensed event interval is lessthan a tachyarrhythmia detection interval; increase a tachyarrhythmiainterval count in response to determining that the sensed event intervalis less than the tachyarrhythmia interval; determine that thetachyarrhythmia interval count reaches a threshold number of intervalsto detect tachyarrhythmia; and detect a tachyarrhythmia in response tothe tachyarrhythmia interval count reaching the threshold number ofintervals to detect tachyarrhythmia; and the therapy delivery circuit isconfigured to deliver an electrical stimulation therapy in response tothe control circuit detecting the tachyarrhythmia.
 12. A method,comprising: receiving a cardiac electrical signal; determining astarting value of a cardiac event sensing threshold; determining asecond value of the cardiac event sensing threshold, the second valuebeing less than the starting value; setting a sense delay interval;starting a blanking interval that expires earlier than the sense delayinterval; holding the cardiac event sensing threshold constant at thestarting value from an expiration of the blanking interval to anexpiration of the sense delay interval; detecting the expiration of thesense delay interval; at the expiration of the sense delay interval,setting the cardiac event sensing threshold to the second value to applythe second value of the cardiac event sensing threshold starting fromthe expiration of the sense delay interval; sensing a cardiac eventattendant to a myocardial depolarization in response to the cardiacelectrical signal crossing the cardiac event sensing threshold; andgenerating a sensed event signal in response to sensing the cardiacevent.
 13. The method of claim 12, wherein setting the sense delayinterval comprises: determining, from the cardiac electrical signal, anR-T interval between an R-wave and a T-wave; and setting the sense delayinterval to be longer than the R-T interval.
 14. The method of claim 12,wherein determining the starting value of the cardiac event sensingthreshold comprises: determining a first amplitude of a T-wave from thecardiac electrical signal; and determining the starting value based onat least the first amplitude of the T-wave.
 15. The method of claim 14,wherein determining the starting value of the cardiac event sensingthreshold comprises: determining a second amplitude of an R-wave fromthe cardiac electrical signal; determining a ratio of the firstamplitude to the second amplitude; and determining the starting value ofthe cardiac event sensing threshold based on the ratio of the firstamplitude to the second amplitude.
 16. The method of claim 15, whereindetermining the starting value of the cardiac event sensing thresholdcomprises: determining a percentage that is greater than the ratio ofthe first amplitude to the second amplitude; starting the blankinginterval in response to the sensing circuit generating a previous sensedevent signal; determining a peak amplitude of the cardiac electricalsignal during the blanking interval; determining the percentage of thepeak amplitude; and determining the starting value of the cardiac eventsensing threshold to be the percentage of the peak amplitude.
 17. Themethod of claim 14, wherein determining the starting value of thecardiac event sensing threshold comprises: determining a minimum sensingthreshold limit based on the first amplitude of the T-wave; and settingthe starting value of the cardiac event sensing threshold to be greaterthan the minimum sensing threshold limit.
 18. The method of claim 12,wherein determining the second value of the cardiac event sensingthreshold comprises: determining a first amplitude of an R-wave from thecardiac electrical signal; determining a second amplitude of a P-wavefrom the cardiac electrical signal; determining a ratio of the secondamplitude to the first amplitude; and determining the second value ofthe cardiac event sensing threshold based on the ratio of the secondamplitude to the first amplitude.
 19. The method of claim 18, whereindetermining the second value of the cardiac event sensing thresholdcomprises: determining a percentage that is greater than the ratio ofthe second amplitude to the first amplitude; starting the blankinginterval in response to the sensing circuit generating a previous sensedevent signal; determining a peak amplitude of the cardiac electricalsignal during the blanking interval; determining the percentage of thepeak amplitude; and determining the second value of the cardiac eventsensing threshold to be the percentage of the peak amplitude.
 20. Themethod of claim 12, further comprising: starting the blanking intervalin response to a previous sensed event signal generated by the sensingcircuit; determining the starting value of the cardiac event sensingthreshold during the blanking interval; detecting an expiration of theblanking interval; and setting the cardiac event sensing threshold tothe starting value in response to the expiration of the blankinginterval.
 21. The method of claim 12, further comprising: starting apacing escape interval in response to the sensed event signal; detectingan expiration of the pacing escape interval; and generating a pacingpulse in response to the expiration of the pacing escape interval. 22.The method of claim 12, further comprising: determining a sensed eventinterval in response to the generated sensed event signal; determiningthat the sensed event interval is less than a tachyarrhythmia detectioninterval; increasing a tachyarrhythmia interval count in response todetermining that the sensed event interval is less than thetachyarrhythmia interval; determining that the tachyarrhythmia intervalcount reaches a threshold number of intervals to detect tachyarrhythmia;and detecting a tachyarrhythmia in response to the tachyarrhythmiainterval count reaching the threshold number of intervals to detecttachyarrhythmia; and delivering an electrical stimulation therapy inresponse to detecting the tachyarrhythmia.
 23. A non-transitorycomputer-readable medium storing instructions, which when executed by acontrol circuit of a medical device cause the device to: receive acardiac electrical signal; determine a starting value of a cardiac eventsensing threshold; determine a second value of the cardiac event sensingthreshold, the second value being less than the starting value; set asense delay interval; start a blanking interval that expires earlierthan the sense delay interval; hold the cardiac event sensing thresholdconstant at the starting value from an expiration of the blankinginterval to the expiration of the sense delay interval; detect anexpiration of the sense delay interval; at the expiration of the sensedelay interval, set the cardiac event sensing threshold to the secondvalue to apply the second value of the cardiac event sensing thresholdstarting from the expiration of the sense delay interval; sense acardiac event attendant to a myocardial depolarization in response tothe cardiac electrical signal crossing the cardiac event sensingthreshold; and generate a sensed event signal in response to sensing thecardiac event.